Suggestion: Health-check example

Async could provide a health check as an example since it is simple enough to implement and useful in general. Constraints

* Ping the server periodically (can be a http endpoint).
* Use only one timer.
* The timer should not be cancelled.
* On expiration the timer should exit with success.

The interface could look like

```cpp
co_await health_check(connection, std::chrono::seconds{1});
```